The exemplary embodiment relates to color control of a discharge lamp during dimming. It finds particular application in connection with a ballast circuit for driving a high intensity discharge lamp which imposes pulses on a dimming waveform to approximate a lamp color exhibited during normal operation.
Discharge lamps produce light by ionizing a vapor fill material, such as a mixture of rare gases, metal halides, and mercury, with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. During operation, a voltage is applied across the electrodes, and a gas discharge occurs in the ionized fill between the electrodes.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including indoor lighting, highway and street lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. Ceramic metal halide discharge lamps, which are characterized by high efficacy and superior color rendering index (CRI), are now widely used for general lighting.
A ballast circuit provides a current waveform that maintains a suitable lamp operating voltage/current during lamp operation. While a low-pressure gas discharge lamp is typically operated with resonant current, i.e. current having a sine-shaped waveform, a high-pressure discharge lamp is typically operated by supplying commutating DC current. An electronic ballast or driver for such a lamp typically comprises an input for receiving AC mains power, a rectifier for rectifying the AC mains voltage to a rectified DC voltage, and a commutator for regularly changing the direction of this DC current. Typically, the commutator operates at a frequency of about 100 Hz. The lamp is operated at a substantially constant current intensity with the lamp current regularly changing its direction within a very brief time (commutating periods). The waveform may thus approximate a square wave.
HID lamps, and in particular, ceramic metal halide discharge lamps used for general lighting, are generally operated at full rated lamp power. This is because the correlated color temperature (CCT) and hue of the lamp change significantly at lower power. At lower power, the lower temperature of the discharge vessel causes a reduction in the vapor pressure of the metal halide fill in the arc discharge chamber, resulting in significant changes of the CCT of the lamp.
Due to the ever increasing cost of energy and increased interest in energy-conserving lighting systems, there is an interest in providing metal halide discharge lamp systems with the capability of operating at less than full power, referred to as dimming. Ballast circuits have been developed which apply a DC waveform during dimming, as disclosed, for example, in US Pub. No. 20070090769 to Collins, et al. Another method involves varying the duty cycle, as disclosed for example in U.S. Pat. No. 7,049,768. One disadvantage of such methods is that the waveform has a distinct DC component and the electrodes are heated asymmetrically. To compensate for the effect of uneven heating on the electrode life, the two electrodes may be constructed differently. However, this can lead to increased production costs and more complex processing.
There remains a need for a ballast circuit capable of driving an HID lamp in a lower power mode which overcomes the above-referenced problems, and others.